Electroluminescent displays are used as light emitting type electronic display device (ELD). As the constituting element of the ELD, an inorganic electroluminescent element (inorganic EL element) and an organic electroluminescent element (organic EL element) are cited. The inorganic EL element has been used as a planar light source though high alternative voltage is required for driving such the light emitting device.
The organic EL element is an element having a light emission layer placed between a cathode and an anode, in which electrons and positive holes are injected into the light emission layer and excitons are generated by recombination of them, and light (fluorescence or phosphorescence) is emitted on the occasion of quenching of the excitons. Such the device is noted because which can emit light by application of a voltage of several to several tens volts, and has wide viewing angle and high visibility since it is a self light emission type, and is completely solid state thin device suitable for space saving and portable appliance.
It is important character of the organic EL element that the element is a planar light source different from ordinary practically used principal light sources such as light emission diodes and cold cathode ray tubes. As the use for effectively utilizing such the character, illuminating light source and backlight for various displays are cited. Particularly, such the element is suitably used for the backlight of liquid crystal full color display which is strongly demanded in recent years.
The element is used as a source of white light when the element is used as the backlight of the full color display. The following methods are applicable for obtaining white light by the organic EL element; a method in which plural light emission materials emitting two kinds of light in additive complementary colors are used in an element so as to emit white light by color mixing (Patent Document 1), a method in which plural materials emitting light of blue, green and red colors for obtaining white light. It is preferred a method employing plural materials emitting light of blue, green and red colors in view of color reproduction, reducing light loss by a color filter and so on.
It has been reported to obtain blue, green and red light that a white light emission organic EL element can be obtained by doping the fluorescent materials reach emits blue, green or red light with high efficiency as the light emission material; cf. Patent Document 2 for example.
moreover, development of phosphorescent light emission material is made progress since an element showing high luminance light emission can be obtained; cf. Non-Patent Documents 1 to 3 for example. Such the effect causes that the light emission efficiency of the phosphorescent light emission material can be raised for 4 times in maximum of that of the fluorescent light emission material because the upper limit of internal quantum efficiency becomes 100% by the later-mentioned formation ratio of the singlet to triplet exciton and the internal conversion from the singlet exciton to the triplet exciton compared with the case of the light emission in usual fluorescent material in which the light is generated from singlet excited state and the formation probability of the light emission exciton species is 25% since the formation ratio of the singlet exciton to triplet exciton is 1:3.
Patent Document 4 discloses that a white light element is obtained by incorporating emission dopants emitting light of blue, green, and red colors in a single emission layer. However, the amount of emission dopants incorporated in the emission layer is at most 3% by weight even when the 3 types thereof are combined. Accordingly, such a small amount results in insufficient luminance.
Further, there is a method of increasing the dopant content to realize further enhanced emission luminance. However, in this case, there have been noted problems such that the drive voltage is relatively high and the power efficiency is decreased.
Patent Document 5 discloses that enhanced luminance and high efficiency can be realized by simultaneously using a plurality of emission dopants emitting light of similar colors in a single emission layer. However, an emission spectrum produced by a combination of dopants featuring different spectrum shapes results in a disadvantage in color reproduction due to a broadened spectrum.
In Unexamined Japanese Patent Application Publication Nos. 2004-006165 and 2004-235168, there is cited an example in which with regard to an element, which realizes white emission color by employing the three colors of blue, green, and red, a light emitting material in an adjacent layer is doped as an auxiliary light emitting substance. However, a recombination region of electrons and positive holes shifts due to variation of the drive current, whereby the object is to color-compensate for a varied emission color, resulting in no contribution to improving power efficiency.    Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter also referred to as JP-A) No. H07-041759    Patent Document 2: JP-A No. H06-207170    Patent Document 3: U.S. Pat. No. 6,097,147    Patent Document 4: JP-A No. 2001-319780    Patent Document 5: JP-A No. 2003-068465    Non-Patent Document 1: M. A. Baldo et al., Nature, Vol. 395, pages 151-154 (1998)    Non-Patent Document 2: M. A. Baldo et al., Nature, Vol. 403, No. 17, pages 750-753 (2000)